fix #97: install nix config correctly for single-user- mode

lib/install-nix.sh

@@ -38,8 +38,7 @@ if [[ $OSTYPE =~ darwin || -e /run/systemd/system ]]; then
 else
   # "fix" the following error when running nix*
   # error: the group 'nixbld' specified in 'build-users-group' does not exist
-  mkdir -m 0755 /etc/nix
-  echo "build-users-group =" > /etc/nix/nix.conf
+  add_config "build-users-group ="
 fi
 
 if [[ $INPUT_INSTALL_OPTIONS != "" ]]; then